(1) Field of the Invention
The invention is an unmanned underwater vehicle that can function as an acoustic vector sensor.
(2) Description of the Prior Art
It is known that a propagating acoustic plane wave in water will cause fluid particles to move in an oscillatory motion. A “fluid particle,” as the term relates to the present invention, is a small volume of fluid surrounding a point where averaged properties (e.g., velocity, temperature, etc.) can be analyzed with continuum mechanics. An acoustic vector sensor measures the particle motion via an accelerometer and combines the motion measurement with a hydrophone in order to obtain a high degree of directionality in a relatively small package. Based on these advantages, acoustic vector sensors have become an active area of research.
An outgrowth in the research of vector sensor technology is that if an object in water is neutrally buoyant and small compared to a wavelength (if the acoustic wavelength is at least ten times larger than the object's representative length scale); the object will respond as a fluid particle in the sense that an acoustic plane wave will cause the object to move back and forth with the same oscillatory motion induced in the surrounding water.
At low frequencies, unmanned underwater vehicles (UUVs) are typically small in measurement as compared to an acoustic wavelength. For example: at a frequency of 100 Hz in water, the wavelength is fifteen meters. This wavelength is large compared to the diameter of almost all known UUVs and is even large compared to the length of many UUVs. If the UUV is also neutrally buoyant in water; the UUV will assume the same motion as the neighboring fluid particles induced by the acoustic fields propagating in the water in a direction transverse to the UUV axis. Thus, the UUV itself can function as an accelerometer for the purposes of acting as an acoustic vector sensor under these conditions.
In the prior art of sensor technology; Glenning (U.S. Pat. No. 6,046,963) discloses an undersea vehicle incorporating a hull array in a stowed position. Sensors are joined to analysis circuitry within an inner hull. The sensors can be either velocity sensors or pressure sensors operating on piezoelectric, optical or magneto-strictive principles or the like. The hull array is slidably mounted at each side to guide track sets. Each guide track has an outer track and an inner track.
In Cray et al. (U.S. Pat. No. 6,697,302) an underwater acoustic receiver is provided that measures pressure. Acoustic particle acceleration being sensed by each of the accelerometers (which can be converted to acoustic velocity by taking the time derivative) is obtained by taking the average of the acceleration along a given axis. For example: the x-acceleration component (denoted “u” in terms of velocity) is obtained by summing accelerometer outputs and dividing by two. The acceleration components are obtained in a similar manner.
In Houston et al. (U.S. Pat. No. 6,972,678), a schematic depicts a planar waveguide formed on the outer surface of a hull of a vessel. The waveguide comprises an outer dielectric layer, an optional metal coat, an inner dielectric layer and the outer surface of the hull.
In Hickling (U.S. Pat. No. 7,054,228), a method and apparatus for locating and quantifying sound sources using an array of acoustic vector probes is disclosed. The set of sound-intensity vectors measured by the array provides a set of directions to a sound source whose approximate spatial coordinates are determined using a least-squares triangulation formula. The sound intensity vectors also determine sound-power flow from the source.
In Cray et al. (U.S. Pat. No. 7,106,658), a single directional sensor that can be positioned on an underwater or surface vehicle is disclosed. A transponder radiates a coded acoustic signal. The signal is received at the sensor on the vehicle. A sensor processor is also positioned on the vehicle. The sensor processor includes a clock synchronized with a source processor clock. The sensor processor calculates distance between the transponder and the sensor using the one-way time delay from signal transmission and the speed of signal propagation through the environment.
In Abdi (U.S. Pat. No. 7,505,367) vector components of an acoustic field may be measured using devices including, but not limited to, transducers, receivers and vector sensors. Measurements of the scalar components of the acoustic field may be made using devices which include, but are not limited to, pressure sensors, transducers, hydrophones, omni-directional hydrophones, directional hydrophones and/or any other devices that achieve the same or similar functionality. Recovering information from the vector components of the acoustic field is not limited to any particular sensor type; any device capable of measuring a vector component of the acoustic field suffices.
In Naluai et al. (U.S. Pat. No. 7,536,913), a probe is disclosed that can be directly mounted to an external support structure via a central support rod at a desired elevation measurement point and oriented in a desired measurement direction. Combinations of the various signal output of the probe yield accurate measurements of the vector field of the acoustic intensity.
In Ruffa (U.S. Pat. No. 7,679,999) a bow dome acoustic sensor assembly is disclosed that includes a forward-most outer hull portion of the submarine and surface ship—known as the “bow dome”. An acoustic panel is mounted on a pressure hull portion via acoustically isolating supports. An after surface of the acoustic panel is provided with optical properties which permit analysis of light from a laser.
Donskey et al. (U.S. Pat. No. 8,085,622) illustrates an ultra low frequency acoustic vector sensor; the acoustic sensor is adapted to measure ultra low frequency liquid particle oscillations when positioned in a body of water. More particularly, the acoustic sensor includes a spherically-shaped housing which has a liquid-tight compartment or horn positioned centrally therein.
Deng (U.S. Pat. No. 8,638,956) illustrates an exemplary buoyant object of an acoustic velocity microphone shown in relation to an acoustic wavelength. The feature size of the buoyant object may be smaller than the wavelength of an acoustic wave. The buoyant object follows the movement of the acoustic particle of the acoustic wave passing thru the buoyant object. In other words, the velocity of the buoyant object is the same as or similar to the particle velocity of the acoustic wave.
Stacey et al. (U.S. Pat. No. 8,385,155) discloses a digital acoustic sensor system comprising an acoustic sensor that is configured to detect an underwater acoustic signal and form an analog signal that is proportional to the underwater acoustic signal. In another embodiment, the acoustic sensor can be an accelerometer configured to sense a change in velocity caused by an underwater acoustic signal. An acoustic vector sensor, such as a hydrophone vector sensor, can be used to measure the direction of the acoustic signal.
The preceding patent references are general approaches for realizing a vector sensor, in some cases not limited to any particular sensor type. The references teach a situation different from an underwater vehicle that can be made neutrally buoyant and is often smaller than an acoustic wavelength. As such, a novel approach would be to use the entire underwater vehicle to emulate an underwater acoustic sensor. Furthermore, the prior art does not teach the use of the accelerometers primarily employed by a UUV for inertial navigation that can also determine acceleration measurements necessary to operate as an acoustic vector sensor.